Once formation of semiconductor devices and interconnects on a semiconductor wafer (substrate) is completed, the semiconductor wafer is diced into semiconductor chips, or “dies.” Functional semiconductor chips are then packaged to facilitate mounting on a circuit board. A package is a supporting element for the semiconductor chip that provides mechanical protection and electrical connection to an upper level assembly system such as the circuit board. One typical packaging technology is Controlled Collapse Chip Connection (C4) packaging, which employs C4 balls each of which contacts a C4 pad on the semiconductor chip and another C4 pad on a packaging substrate. The packaging substrate may then be assembled on the circuit board.
Each C4 pad is a contiguous metal pad typically formed out of the last metal layer of a metal interconnect structures during a semiconductor manufacturing sequence. Each C4 pad is large enough to accommodate the bottom portion of a C4 ball. A bond pad structure refers to a structure containing such a C4 pad and attached structures underneath.
A metal structure such as a C4 ball comprises a lattice of metal ions and non-localized free electrons. When electrical current flows through a C4 ball, the metal ions are subjected to an electrostatic force due to the charge of the metal ion and the electric field to which the metal ion is exposed to. Further, as electrons scatter off the lattice during conduction of electrical current, the electrons transfer momentum to the metal ions in the lattice of the conductor material. The direction of the electrostatic force is in the direction of the electric field, i.e., in the direction of the current, and the direction of the force due to the momentum transfer of the electrons is in the direction of the flow of the electrons, i.e., in the opposite direction of the current. However, the force due to the momentum transfer of the electrons is generally greater than the electrostatic force. Thus, metal ions are subjected to a net force in the opposite direction of the current, or in the direction of the flow of the electrons. The mass transport caused by the electrical current, or the movement of the conductive material due to electrical current, is termed electromigration in the art.
Once a void is formed by electromigration in a C4 ball, the current density at a critical conduction path, or the “hot spot,” increases because the void reduces the area of the conductive path. The higher current density thus accelerates the electromigration process, thereby making the void grow. The vicious cycle of the mutually reinforcing increase in the size of the void and the current density eventually leads to an effective disconnect in the electrical path, causing an electromigration failure. The electromigration failure is a major reliability concern for the C4 balls.
Therefore, there exists a need to reduce the electromigration failure of C4 balls to enhance the reliability of C4 connections.